1. Field of the Invention
The present invention relates to a new homogeneous cytosolic binding (HCB) protein which binds FK-506 but not cyclosporine A, is stable to heating at 56.degree. C. for 30 minutes, and has a molecular weight of about 10-12 kilodaltons. The (partial) amino terminal amino acid sequence of the HCB protein is: H.sub.2 N-Gly-Val-Gln-Val-Glu-Thr-Ile-Ser-Pro-Gly-Asp-Gly-Arg-Thr-Phe-Pro-Lys-Arg- Gly-Gln-Thr-X-Val-Val-His-Tyr-Thr-Gly-Met-Leu-Glu-Asp-Gly-Lys-Lys-Phe-Asp (wherein X is undefined). The HCB protein has peptidyl-proline isomerase enzymatic activity.
2. Brief Description of Disclosures in the Art
In 1983, the U.S. FDA licensed cyclosporine, and extremely effective anti-rejection drug that revolutionized the field of organ transplant surgery. The drug acts by inhibiting the body's immune system from mobilizing its vast arsenal of natural protecting agents to reject the transplant's foreign protein.
As effective as the drug is in fighting transplantation rejection, it suffers drawbacks in causing kidney failure, liver damage and ulcers which in many cases can be very severe.
EPO Publication No. 0184162 to Fujisawa, describes a new macrolide immunosuppressant FK-506 which is reputed to be 100 times more potent than cyclosporine. The macrolide is produced by fermentation of a particular strain of Streptomyces tsukuhaensis. Also described is the closely related macrolide immunosuppressant FK-520, produced by S. hygroscopicus subsp. yakushimaensis.
FK-506 shares a number of immunosuppressive properties with the cyclic peptide, cyclosporine A, though 10-100 times more potent in this regard. These similarities suggest that both agents may share a similar mechanism of action at the biochemical level.
For example, Cyclosporine A is known to bind with the cytosolic protein, cyclophilin, as described in SCIENCE, Vol. 226, pp. 544-546 (Nov. 1984) by R. E. Handschumacher et al, which is thought to mediate the immunsuppressive effects of cyclosporine A in cells. Cyclophilin has also been shown to possess an enzymatic activity, which catalyzes the cis-trans isomerization of peptidyl prolyl bonds, as described in Nature. Vol. 337, pp. 473-475 and pp. 476-478 by N. Takahashi et al. and Fischer et al., respectively.
Further, the article by V. Warty et al in Transplantation Vol. 46, No. 3, pp. 453-455 (September 1988) present data suggesting that FK-506 also binds a cytosolic protein with an apparent molecular weight of about 18-19 kilodaltons which is similar to the protein that also binds cyclosporine.
What is needed in the art is the isolation and identification of that particular FK-506 binding protein in order to help delineate the FK-506 mechanism of therapeutic action in the cell.